Cardiac contraction is mediated by a transient increase in calcium inside the cytoplasm. At the cellular level, the increase in calcium is due to calcium coming from the extracellular milieu and calcium stored in the sarcoplasmic reticulum. It is well known that in mammalian cardiac cells most of the calcium for cardiac contraction (~70%) is from the calcium stored in the sarcoplasmic reticulum. In fish cardiac cells, the relative contribution of calcium coming from the extracellular space versus sarcoplasmic reticulum is not clear. This study addresses this point. Trout ventricular myocytes were enzymatically isolated according to the UK law. Calcium current (ICa) was recorded using the whole-cell configuration of the patch clamp technique with Na- and K-free solutions to avoid contaminating currents. Using different calcium buffers, inactivation of ICa was used as an index of sarcoplasmic reticulum calcium release. Calcium concentration in the cytoplasm was also recorded using calcium fluorescent dye (fura-2 AM). With a slow Ca buffer (EGTA 2mM in the pipette solution), ICa inactivated slowly: the time to reach 37% of peak current (T37) was 27.1 ± 1.8 ms (n=37). With a fast Ca buffer (BAPTA 10 mM), ICa decay was similar to the decay in the presence of EGTA (T37: 30.3 ± 2.4 ms, NS, t-test, n=20). In contrast, during beta-adrenergic stimulation, inactivation of ICa in the presence of EGTA (T37: 11.6 ± 1.7 ms, n =18) was significantly faster than in presence of BAPTA (T37: 27.3 ± 1.6 ms, n =12, p<0.05, t-test), indicating the presence of SR calcium release. In the same way, calcium concentration in the cytoplasm recorded using fura-2 AM during electrical field stimulation was significantly larger during beta-adrenergic stimulation compare to control condition (F340/380= 0.026 ± 0.005 RU in control condition versus 0.046 ± 0.010 RU during beta-adrenergic stimulation, n=10 and 10 respectively). Caffeine-induced transients indicates that a small, but significant, increase in calcium concentration in the cytoplasm (an index of sarcoplasmic reticulum calcium load) was present after beta-adrenergic stimulation (F340/380= 0.080 ± 0.022 RU before beta-adrenergic stimulation versus 0.112 ± 0.028 RU after beta-adrenergic stimulation, n=10 and 10 respectively). At rest, fish cardiac cells are not using calcium stored in the sarcoplasmic reticulum. However, during a stress (e.g. beta-adrenergic stimulation), calcium from the sarcoplasmic reticulum plays an important role in cardiac contraction. We propose that the calcium stored in the sarcoplasmic reticulum of fish cardiac cells acts as a safety mechanism, allowing greater contraction when needed by the environment.